Conventionally, the following methods are known for detecting a regular image signal (mentioned "a sharp signal" hereinafter) and an unsharp signal in a laser beam scanner system.
One of the methods comprises processes of scanning an original picture with two beams for obtaining the sharp and the unsharp signals, obtaining beams informed with picture data by sole photo-sensor, and detecting both signals from the beams. In this method, as disclosed in U.S. Pat. No. 4,080,634, one of the beams for obtaining the sharp and the unsharp signals is modulated and the beams must be coaxially arranged in order to discriminate both signals. Then the beams reflected at or transmitted through the original picture are detected by the sole photo-sensor, and the sharp and the unsharp signals are discriminated by decoding the detected signal.
However this kind of method has a defect that both signals cannot completely be discriminated even when the decoding work is done in perfect condition.
In addition, a modulation frequency for said beam must be higher than a scanning frequency, which leads to unstable modulation in company with increase of the scanning frequency.
Another method employs a way of detecting a sharp and an unsharp signals by respective photo-sensors. In this method, as disclosed in Japanese Patent Publication No. 54-22122, two beams of different wavelength are used for obtaining the sharp and the unsharp signals, for example a red beam is used for the former signal and a blue beam is used for the latter signal, and the photo-sensors have distinct color filters to detect respective beams of proper wavelength.
In this kind of method, no such defects as in the method mentioned at first exists, but another problem takes place. That is, it is apprehended that unexpected detail-emphasis signals (unsharp mask signals) are detected on the red or blue portions of an original picture when it is colored.
Moreover, as two photo-sensors are used, there is a difficulty to pair them up.
The other method employs a way of digitally computing image data of an original picture to obtain a detail emphasized signal as disclosed in U.S. Pat. No. 4,319,268. However this method has difficulties that a memory is necessary to store image data of a scanning line along with of the scanning lines before and behind the scanning line, and that a complicated computer module is required to digitally compute the image data on a pixel scale.